Microwave antenna



i v'r March 21, 1950 QSTEINBERGER 2,501,105

MICROWAVE ANTENNA Filed Nov. 2'7, 1945 &

9 e \\\T\\\ 1 l0 7' AI T F|G 2 INVENTOR.

i A JACK STEINBERGER B ATTORNEY Patented Mar. 21, 1950 NITED STATES MICROWAVE ANTENNA War Application November This invention relates to antennas, and more particularly tomeans comprising a waveguide antenna which may be utilized as a linev source of electromagnetic energy.

In the radiation of electromagnetic energy it is often desirable to utilize a waveguide antenna array, in which a plurality of branching elements may be employed to extract power from the main waveguide and radiate the electromagnetic energy thus extracted into space, or toward a reflecting surface. It is highly desirable that each branching element of a waveguide fed array introduce the least possible reflection or disturbance in the main waveguide, since reflections introduced by the branching elements lead to an increased standing wave ratioand a consequent loss of power. The present invention is an improvement upon a waveguide fed array disclosed in the copending application of Edward M. Purcell, Serial Number 608,297, filed August 1, 1945, entitled Antenna.

It is contemplated in the invention to feed electromagnetic energy into a rectangular tapered waveguide from which extend branching waveguides in a manner similar to that disclosed in the above mentioned application of Edward M. Purcell.

Among the objects of the present invention are to provide a Waveguide fed array which will minimize reflections from the branching elements thereof into the waveguide; to provide a waveguide fed array whose operating frequency may vary considerably without appreciable disadvantages in operation; and to provide a conveniently used waveguide fed array whose characteristics including directional effects, operating frequency band width, and reflection from the branching elements thereof, are improved over those of the array disclosed inv the hereinbefore mentioned application.

Further objects, novel features and advantages of the invention will be apparent from the description contained herein, wherein reference is made to the accompanying drawings illustrating a preferred embodiment of the invention, in which:

Fig. 1 is a side View of an entire array;

Fig. 2 is a cross-sectional view on the line 2--2 indicated in Fig. 1;

Fig. 3 is an enlarged cross-sectional View of a central portion of the structure taken on the line 33 of Fig. 1, illustrating branching waveguides of the array; and

Fig. 4 is a perspective view, partially in section, of a ortionof the array shown. in Fig. 1.

7 Claims.

1945, Serial No. 631,169

Referring now to Figs. 1, 2 and 4, a rectangular feed waveguide 5 is provided for introducing energy into a main Waveguide 6. Feed waveguide 5 is located centrally along main waveguide 6 and is substantially normal'to the longitudinal axis thereof. Waveguide 6 is rectangular and is tapered from the center portion thereof, at which waveguide 5 connects, toward each end so that its smaller dimension decreases from width A at the center to width A at the ends. Branching waveguides are inserted along one broad wall 1' of waveguide 6 whose width 13 is uniform along the guide. These branching waveguides, communicate with a flared aperture or waveguide 8 extending along the structure. Waveguide 8 here functions substantially as a line source for illuminating a parabolic cylinder reflector, or for radiating energy directly into space. Waveguide 8 may be enclosed by a dielectric sheet 9 to protect the inner operative portion of the waveguide structure from the elements. If pressurization of the structure is desired, end members may be provided, and a gasket may be inserted between dielectric sheet 9 and the framework of waveguide 8.

Referring now to Fig. 3 which displays a cross section of a few of the branching, elements of the waveguide fed array, branch waveguides Ill and ID on one side, and H and II on the other side, extend symmetrically from a central transverse plane [2 of the array. Branch waveguides I0 and Ill are symmetrically spaced alternately on each side of a center line [3 which represents the longitudinal central plane passing through broad wall E of waveguide 6. Likewise, branch waveguides II and H are symmetrically spaced alternately on each side of the center line 13. The axes of the series of branching waveguides It and H), II and H, all extend transversely to broad wall 1. Considering the shape of the apertures formed by the branching guides in broad wall I, the cross-section of each aperture may be described as corresponding to that area, lying to one side or the other of center line l3, formed by a tipped rectangle having one corner on center line 13. Furthermore, adjacent rectangles are tipped in opposite directions, and overlap so that adjacent branching elements l0 and I0 form a continuous opening in wall I, as do adjacent branching elements ll and I I. The description of these rectangles may be better understood by reference to Fig. 3, in which dotted lines l4, l5, l6 and I! complete such rectangles upon adjacent branch waveguides I0 and I0.

Another. way in which the openings may be described is to consider them as trapezoidal in shape. Taking branching element H as an example, a portion of line l3 forms one side of the trapezoid, and another side is formed by a line I8 which extends at a small angle to center line [3 but does not intersect therewith. Line 19 extends perpendicularly to line [8 to intersect center line l3, and the trapezoidal figure of the opening is completed by line 20 (which is parallel to line l9 and likewise perpendicular to line I8) which extends a short distance to line l3.

The advantage in so arranging the branching waveguides lies in the fact that the characteristic impedance of main waveguide 6 is then better matched than by branching rectangular waveguides such as illustrated in the prior application of said Edward M. Purcell. By suitably choosing the dimensions of the branching waveguides, their impedance may be made to substantially match the characteristic impedance of main waveguide 6, so that substantially no reflection is produced in main waveguide 6.

An explanation which may be advanced for the difference in the matching characteristics of the two forms of branching guides is that each of the Purcell branch guides may be represented, in an equivalent circuit, as a substantially resistive impedance across a line, whereas each of the branch guides of the present invention may be represented as a T network of impedances. The T network by proper adjustment, may therefore provide a perfect match to the characteristic impedance of the line. Because of the improved match obtainable with the branch guides of the present invention, the frequency response of the antenna array is improved.

It may be further noted that in the Purcell array, resonance phenomena have been observed at an operating frequency at which the branch guides are spaced at half guide-wavelength intervals. This resonance phenomenon causes the radiation pattern to deteriorate. Such an undesirable effect is absent in the present invention.

By center-feeding main waveguide 6 and tapering the main waveguide uniformly at a suitable angle, each of the branching elements l0, ill, II and II will withdraw substantially equal amounts of power. The alternate branching elements are spaced one guide-wavelength apart along the center axis 13. As adjacent branching elements Ill and Ill are on opposite sides of the center axis [3, and spaced along that axis at half guide-wavelengths from each other, they will radiate power substantially in phase. Branching elements H and II similarly radiate power substantially in phase. The arra of the embodiment here disclosed is symmetrical with respect to central transverse plane l2, and energy radiated from the two sections of waveguide 8 which lie on each side of plane l2 may be made to be substantially in phase. It has been found that this over-all in-phase condition may be achieved by making the two central elements more nearly triangular in cross-section, and by separating these central elements by a relatively thin dividing member or septum 2i to suitably space them. The energy from all the brancing elements of the waveguide fed array will then emanate from flared aperture 8 in a highly directive beam having a straight line phase front.

It might appear that if the operating frequency were changed substantially, two directive lobes would result in the radiation pattern emanating from flared aperture 8. However, one advantage of the structure here disclosed, resulting partially from the symmetry about central axis I2, is the fact that even though the operating frequency is changed substantially, the energy radiation remains strongest in the direction normal to the array. Lobing (production of divergent minor beams) occurs only when a comparatively great change from the normal operating frequency is made. The antenna structure therefore radiates energy which does not depart substantially from the desired directive characteristics, even though the operating frequency is not precisely maintained. This is a particular advantage at short wavelengths where it may be diflicult to maintain a precise operating frequency.

As an aid in the construction of similar arrays, the dimensions of the branch waveguides shown in the drawing are here given and expressed in terms of A, the free-space wavelength of the mid-frequency of a chosen band of operating frequencies. In the embodiment of the invention here illustrated, the dimension a, Fig. 3 indicating the thickness of central septum 2|, is .031 Measured from central plane l2 to the far corner of each of central branch guides l0 and l I, the distance I is .69 and to the near corner of each of the adjacent branch guides l0 and H, the distance e is .5l7\. Thus, adjacent guides IO and I 0' overlap by substantially .18 as do adjacent guides H and II. Branch guides it are spaced uniformly from each other along center line l3 by a distance b equal to 1.24)., as are the branch guides l0, II and H. The distance I) substantially corresponds to one wavelength in the main guide 6 at the mid-frequency of the chosen band of operating frequencies. The inclined side of each trapezoidal opening makes an angle 0 of 11 to center line I3. Excepting for the central branch guides i0 and H, the inclined side of each branch guide has a length (1 equal to .76) In each of the branch waveguides as seen in Fig. 3, the corner extending farthest from center line I3 is at a distance g therefrom equal to .13 Although the shape and size of central elements In and II do not conform exactly to the other elements, they are substantially the same. Furthermore, in a practical embodiment there are a large number of branch waveguides, and slight departures in the measurements of the central branching elements are therefore of no material eifect. The dimensions herein given are the result of empirical data analyzed to impart to the branch guides the desired impedance as viewed from the main guide.

As is well known in the art, an antenna intended for the radiation of energy may also be used for the reception of energy with similar directive and impedance matching properties, it it is linear, as is the device herein described. The antenna here described may thus be used equally well either for radiation or reception of electromagnetic energy. Although the embodiment herein shown is center-fed, a non-symmetrical array may be built utilizing the impedance matching elements shown.

It will be apparent to those skilled in the art that many variations may be made in the disclosed structure without departing from the 1. An antenna array, including a rectangular main waveguide having broad and narrow walls.

said narrow walls decreasing in dimension linearly with the distance from a central transverse plane of said main waveguide toward each'end thereof, a plurality of waveguides branching from one said broad walland forming apertures therein spaced. symmetrically along said main Waveguide as measured from said transverse plane, each said branching waveguide having a substantially trapezoidal transverse cross-section, and a feed means connected to said main waveguide at said central transverse plane.

2. The array of claim 1 wherein said apertures of said first broad wall, the spacing of said branching Waveguides from said transverse plane being such that said apertures overlap to form overlap to form a continuous opening in said broad wall.

3. An antenna array including a rectangular main waveguide having broad and narrow walls, said narrow walls decreasing in dimension linearly with the distance from a transverse plane of said main waveguide toward an end thereof, a plurality of waveguides branching from one said broad Wall and forming a series of apertures therein along said main waveguide, each branching Waveguide having a substantially trapezoidal transverse cross section, and a feed means connected to said main waveguide at said transverse plane.

i. The array of claim 3 wherein said apertures overlap to form a continuous opening in said broad wall.

5. An antenna array comprising a rectangular main waveguide having broad and narrow walls, said narrow walls decreasing in dimension linearly with the distance from a central transverse plane of said main waveguide toward eachend thereof, a plurality of waveguides branching from a first broad wall and forming apertures therein spaced symmetrically alongsaid main waveguide as measuredfrom said transverse plane, each of said branching waveguides being trapezoidal in transverse cross section and being oriented so that one non-parallel side of each branching waveguide lies along the longitudinal mid-plane a single continuous opening on each side of said transverse plane, and feed means connected to said main waveguide at said transverse plane.

6. An antenna array in accordance with claim 5, said array further comprising a first and a second conductive surface extending from said first broad wall on opposite sides of said longitudinal midplane, said surfaces defining a flaring horn external of said waveguide and normal to said first broad wall.

'7. An antenna array comprising a rectangular waveguide having broad and narrow walls, a flaring electromagnetic horn extending the length of said main waveguide, said electromagnetic horn having the base thereof adjacent to a first broad wall of said waveguide, and a plurality of waveguides branching from said first broad wall of said main waveguide and forming apertures therein, each of said branching waveguides.

having a substantially trapezoidal transverse cross-section wherein said branching waveguides connect said main waveguide to said horn.

JACK STEINBERGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,405,242 Southworth Aug. 6, 1946 FOREIGN PATENTS Number Country Date 840,992 France May 8, 1939 893,606 France Feb. 14, 1944 

